North's next test may be of a miniaturized tactical nuke: Heinonen

A commercial satellite image of the Yongbyon Nuclear Scientific Research Center in North Pyongan Province from Nov. 23. [YONHAP]

A former high-ranking official at the United Nations nuclear watchdog said North Korea would likely focus on testing a miniature nuclear warhead in a possible seventh nuclear test.

Oli Heinonen, a former deputy director at the International Atomic Energy Agency (IAEA) and currently a special fellow at the 38 North research division of the U.S.-based Stimson Center, was quoted in a report by Radio Free Asia (RFA) on Monday saying a nuclear test by Pyongyang could be aimed at checking its progress in miniaturizing nuclear warheads that could be mounted onto tactical ballistic missiles.

But Heinonen, whose work at the IAEA was chiefly concerned with the detection of the misuse of nuclear material or technology, cautioned that Pyongyang likely does not have enough fissile material in stock to manufacture such tactical nuclear weapons.

"Although North Korea's Yongbyon 5-megawatt reactor has been in operation for a long time now, the plutonium reprocessed [there] is older than a year," Heinonen said, referring to the North's main uranium enrichment and reprocessing facility in Yongbyon, North Pyongan Province.

"This plutonium is not suitable for use in the miniaturization of nuclear warheads, so they will have to produce new plutonium."

Yongbyon's 5-megawatt reactor has long been the focal point of previous failed international diplomatic efforts to rein in North Korea's nuclear ambitions. The reactor is believed to be the regime's sole source of spent nuclear fuel for reprocessing and is capable of producing six kilograms of weapons-grade plutonium from spent fuel rods per year.

Although North Korea is believed to have conducted at least one small-scale nuclear test to check its progress in developing tactical nuclear weapons before it suspended testing in 2018, Heinonen theorizes that the regime will push ahead with additional nuclear tests because there are still gaps in its miniaturization knowledge.

While there is no exact definition of tactical nuclear weapons in terms of their range or yield, Heinonen explained that miniaturized nuclear warheads can be used as tactical nuclear weapons, and that tactical nuclear tests usually have an explosive yield of 20 kilotons — roughly equivalent to each of the explosions that destroyed Hiroshima and Nagasaki at the end of the Second World War.

In May, 38 North said that North Korea's Punggye-ri nuclear test site could likely handle explosions with yields up to 120 kilotons, adding that the ongoing restoration of Tunnel No. 3 at the site is likely for the purpose of testing tactical nuclear warheads.

IAEA Director General Rafael Grossi expressed alarm in early September over signs that North Korea is continuing construction on Tunnel No. 3 and 4 at Punggye-ri.

He noted signs that Tunnel No. 3 has been reopened and is prepared for a test.

According to Grossi, the agency has also very recently observed renewed work on the road leading to Tunnel No. 4 and called the site's reopening "deeply troubling."

The restoration of both tunnels at Punggye-ri raises the possibility of more than one nuclear test.

Satellite photography of Punggye-ri taken within the past year shows North Korea has worked to restore access to the underground test site, which was ostensibly demolished in May 2018, when North Korea detonated all four tunnels in a much-publicized event witnessed by foreign media allowed into the country.

BY MICHAEL LEE [lee.junhyuk@joongang.co.kr]